How far from the grating a screen for the second order minima to occur 6.8cm from the central maxima

A grating with 1500 lines/cm has blue light with a wavelength of 490 nm incident on it. a. You measure a diffraction angle of ${12.7}^{\circ }$ to one of the maxima. Which order bright spot is this? b. How far from the grating should you place a screen in order for the second order minima (dark spot) to occur 6.8 cm from the central maxima?
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The equation for diffraction of light is
$d\mathrm{sin}\theta =m\lambda$
where d is the slit width
m is the order of diffraction
$\mathrm{tan}\theta =\frac{\text{distance of second minima from central maxima}}{\text{distance of screen from grating}}=\frac{y}{D}$
The angle of diffraction is 12.7 degree, wavelength is 490 nm. There are 1500 lines / cm. The slit width is

therefore from equation
$m=\frac{6.66×{10}^{-6}\mathrm{sin}{12.7}^{\circ }}{490×{10}^{-9}}$
diffraction angle for order 2 minima is

Now from equation , distance of screen is

a) Order is 3
b) distance of screen from grating is 45.9 cm